Mitogen-Activated Protein (MAP) Kinases
Mitogen-activated protein (MAP) kinases are a family of enzymes which regulate intracellular signaling pathways. MAP kinases are important mediators of signal transduction from cell surfaces to nuclei via phosphorylation cascades. Several subgroups of MAP kinases have been defined and each manifests different substrate specificities and responds to various distinct extracellular stimuli. Thus, the MAP kinase signaling pathways represent common mechanisms for signal transduction by which different extracellular stimuli generate distinct physiological responses inside cells (Egan S E and Weinberg R A (1993) Nature 365:781-783).
Various MAP kinase signaling pathways have been defined in mammalian cells as well as in yeast. In mammalian cells, the extracellular stimuli activating the MAP kinase signaling pathways include epidermal growth factor (EGF), ultraviolet light, hyperosmolar medium, heat shock, endotoxic lipopolysaccharide (LPS), and pro-inflammatory cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1). In the yeast, Saccharomyces cerevisiae, various MAP kinase signaling pathways are activated by exposure to mating pheromone or hyperosmolar environments and during cell-wall construction, sporulation and mitosis.
There are at least three subgroups of MAP kinases in mammalian cells (Derijard B et al (1995) Science 267:682-5), and each subgroup is distinguished by a tripeptide sequence motif. They are extracellular signal-regulated protein kinase (ERK) characterized by Thr-Glu-Tyr, c-Jun amino-terminal kinase (JNK) characterized by Thr-Pro-Tyr, and p38 kinase characterized by Thr-Gly-Tyr. The subgroups are activated by the dual phosphorylation of the threonine and tyrosine by MAP kinase kinases located upstream of the phosphorylation cascade. Activated MAP kinases phosphorylate other effectors downstream ultimately leading to changes inside the cell.
MAP Kinase Subgroup ERK
The ERK signal transduction pathway is activated via tyrosine kinase receptors on the plasma membrane of the cell. When EGF or other growth factors bind to the tyrosine receptors, they, in turn, bind to noncatalytic, src homology (SH) adaptor proteins (SH2-SH3-SH2) and a guanine nucleotide releasing protein. The latter reduces GTP and activates Ras proteins, members of the large family of guanine nucleotide binding proteins (G-proteins). The activated Ras proteins bind to a protein kinase C-Raf-1 and activate the Raf-1 proteins. The activated Raf-1 kinase subsequently phosphorylates MAP kinase kinases which, in turn, activate MAP kinase ERKs by phosphorylating the threonine and tyrosine residues of the ERKs.
ERKs are proline-directed protein kinases which phosphorylate Ser/Thr-Pro motifs. In fact, cytoplasmic phospholipase A2 (cPLA2) and transcription factor Elk-1 are substrates of the ERKs. The ERKs phosphorylate Ser.sub.505 of cPLA2 and cause an increase in its enzymatic activity resulting in an increased release of arachidonic acid and the formation of lysophospholipids from membrane phospholipids. Likewise, phosphorylation of the transcription factor Elk-1 by ELK ultimately results in increased transcriptional activity.
MAP Kinase Subgroup JNK
An analysis of a deduced primary sequence of the two isoforms of JNK, 46 kDa and 55 kDa, reveals that they are distantly related to the ELK subgroup. They are similarly activated by dual phosphorylation of Thr and Tyr, and the MKK4, MAP kinase kinases (Davis R (1994) TIBS 19:470-473). The JNK signal transduction pathway can also be initiated by ultraviolet light, osmotic stress, and the pro-inflammatory cytokines, TNF and IL-1. The Ras proteins may partially activate the JNK signal transduction pathway. JNKs phosphorylate Ser.sub.63 and Ser.sub.73 in the amino-terminal domain of the transcription factor c-Jun which results in increased transcriptional activity.
MAP Kinase Subgroup p38
An analysis of the cDNA sequence encoding p38 shows that p38 is a 41 kD protein containing 360 amino acids. Its dual phosphorylation is activated by the MAP kinase kinases, MKK3 and MKK4. The p38 signal transduction pathway is also activated by heat shock, hyperosmolar medium, IL-1 or LPS endotoxin (Han J et al (1994) Science 265:808-811) produced by invading gram-negative bacteria. The human body reacts to the invading bacteria by activating cells in the immune and inflammatory systems to initiating the systemic response called sepsis. Sepsis is characterized by fever, chills, tachypnea, and tachycardia, and severe cases may result in septic shock which includes hypotension and multiple organ failure.
LPS may be thought of as a stress signal to the cell because it alters normal cellular processes by inducing the release of mediators such as TNF which has systemic effects. CD14 is a glycosylphosphatidyl-inositol-anchored membrane glycoprotein which serves as an LPS receptor on the plasma membrane of cells of monocytic origin. The binding of LPS to CD14 causes rapid protein tyrosine phosphorylation of the 44- and 42- or 40-kD isoforms of MAP kinases. Although they bind LPS, these MAP kinase isoforms do not appear to belong to the p38 subgroup.
Other MAP Kinase Homologs
Recent research (Lee J C et al (1994) Nature 372:739-745) has revealed that a new series of pyridinyl-imidazole compounds, which inhibit LPS-mediated human monocyte IL-1 and TNF-.alpha. production actually work through a pair of closely related MAP kinase homologs, termed cytokine suppressive binding proteins (CSBPs). These compounds are cytokine-suppressive anti-inflammatory drugs (CSAIDs) which prevent phosphorylation and subsequent cytokine biosynthesis. A comparison of fragments of CSBP sequences with those of MAP kinases shows that genes encoding CSBPs are novel although related to protein serine/threonine kinases. It appears that CSBP proteins may be critical for cytokine production during human immune or inflammatory reactions.
Understanding the mechanism for blocking the specific kinase activities may provide a new way of treating inflammatory illnesses. Likewise, a thorough understanding of the various MAP kinase signaling pathways can enable scientists to better understand cell signaling in other developmental and disease processes. Identification of novel MAP kinases provides the opportunity to diagnose or intervene in such disease processes.